Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.095.

The azhan is a call to inform others in specific words that the time for a prayer has begun. It is a call to the congregation, and is an expression of the Islamic practices. It is obligatory or highly preferred. Al-Qurtubi and others have said that the azhan, although it has very few words, covers all essentials of the faith. It begins by proclaiming the greatness of Allah, pointing to His existence and perfection. It mentions His oneness and the denial of polytheism, and it confers the messenger-ship of Muhammad, upon whom be peace. It calls to specific acts of obedience after testifying to Muhammad’s messenger-ship, and it calls to a prosperity which is everlasting, pointing to the return to Allah. Then, in a manner of emphasis, it repeats some of what was already mentioned.